1. Field of the Invention
This invention relates to a refrigerating cycle system wherein the pressure of a refrigerant on a high-pressure heat exchanger side exceeds the critical pressure of the refrigerant, and particularly to such a refrigerating cycle system having carbon dioxide (CO.sub.2) as its refrigerant (hereinafter a CO.sub.2 cycle system).
2. Description of the Related Art
In a CO.sub.2 cycle system, such as the one disclosed in Japanese Published Application No. Hei. 3-503206, refrigerating capacity is controlled via control of pressure on the heat exchanger outlet side.
In this connection, the present applicant has already proposed in Japanese Patent Application No. Hei. 8-11248 a CO.sub.2 cycle system, or supercritical refrigerating cycle system, wherein the aperture of a valve disposed on the heat exchanger outlet side is adjusted in correspondence with the refrigerant temperature thereat.
Specifically, when the refrigerant temperature on the outlet side of the heat exchanger rises, the valve aperture is reduced to raise the refrigerant pressure on the heat exchanger outlet side. When the refrigerant temperature on the heat exchanger outlet side falls, the valve is adjusted to reduce the refrigerant pressure thereat.
However, a so-called variable capacity compressor has certain limitations when applied to a CO.sub.2 cycle system in a vehicle air-conditioning system.
With not only CO.sub.2 cycle systems but also with any refrigerating cycle system using a variable capacity compressor, when the engine speed is relatively high and the mass flow of refrigerant circulating in the refrigerating cycle is large, or when the temperature inside the passenger compartment is stable and the refrigerating capacity required of the air-conditioning system is relatively small, the refrigerant delivery capacity of the compressor is reduced to prevent the refrigerating capacity from becoming excessive.
When a variable capacity compressor and valve having its aperture controlled in correspondence with the refrigerant temperature on the heat exchanger outlet side as in the application mentioned above (Japanese Patent Application No. Hei. 8-11248) are combined, such as when the refrigerant delivery capacity on the compressor side is reduced to prevent the refrigerating capacity from becoming excessively large, the refrigerant pressure on the corresponding heat exchanger outlet side falls.
However, on the valve side, to maintain the refrigerant pressure on the outlet side of the heat exchanger at a pressure (refrigerating capacity) corresponding to the refrigerant temperature on the outlet side of the heat exchanger at that time, the aperture of the valve is reduced to raise the refrigerant pressure on the outlet side of the heat exchanger.
In other words, whereas the refrigerant delivery capacity of the compressor is controlled on the compressor side in correspondence with the required refrigerating capacity, control on the valve side attempts to maintain the refrigerating capacity corresponding to the refrigerant temperature on the heat exchanger outlet side. Thus when these two forms of control are combined, the CO.sub.2 cycle cannot be adequately controlled.